1. Field of the Invention
The present invention relates to a sensor (an MEMS sensor) manufactured by an MEMS (Micro Electro Mechanical Systems) technique and a method of manufacturing the same.
2. Description of Related Art
An MEMS sensor, having been recently loaded on a portable telephone or the like, is increasingly watched with interest. For example, a silicon microphone (an Si microphone) is a typical example of the MEMS sensor.
The silicon microphone includes a silicon substrate. A through-hole is formed on a central portion of the silicon substrate, to penetrate the silicon substrate in the thickness direction. On the silicon substrate, a diaphragm made of polysilicon is arranged on a position opposed to the through-hole. The diaphragm is provided in a state floating up from the silicon substrate (at a small interval from the surface of the silicon substrate), to be vibratile in a direction opposed to the surface of the silicon substrate. A back plate made of polysilicon is arranged on a side of the diaphragm opposite to the silicon substrate. The back plate is opposed to the diaphragm at a small interval. The surface of the back plate is covered with a protective film made of SiN (silicon nitride).
The diaphragm and the back plate constitute a capacitor having the diaphragm and the back plate as counter electrodes. When the diaphragm vibrates by a sound pressure (a sound wave) while a prescribed voltage is applied to the capacitor, the capacitance of the capacitor changes, and voltage fluctuation between the diaphragm and the back plate resulting from the change of the capacitance is output as a sound signal.
In the steps of manufacturing the silicon microphone, a lower sacrificial layer is formed on the silicon substrate, and the diaphragm is formed on the lower sacrificial layer. Thereafter an upper sacrificial layer is formed to cover the overall region of the surface of the diaphragm, and the back plate is formed on the upper sacrificial layer. After the formation of the back plate, the protective film is formed to cover the overall regions of the surfaces of the upper sacrificial layer and the back plate. A through-hole is formed in the protective film, and an etchant is thereafter supplied to the inner side of the protective film through the through-hole, to remove the lower sacrificial layer and the upper sacrificial layer. Thus, the diaphragm floats up from the silicon substrate, while a space of a small interval is formed between the diaphragm and the back plate.
In consideration of the etching selection ratio between the lower and upper sacrificial layers and the protective film etc., SiO2 (silicon oxide) is employed as the material for the lower and upper sacrificial layers and HF (hydrofluoric acid) is employed as the etchant in general. However, a relatively long time (20 to 30 minutes, for example) is required for removing the lower and upper sacrificial layers while the protective film is also exposed to the hydrofluoric acid over the long time, and hence the protective film is disadvantageously etched.